The J3 class battlecruiser was a design study conducted during the Royal Navy's 1921 Fleet modernization programme.

As a follow-on to the Admiral-class battlecruiser, the J3 class incorporated all the lessons learned from the First World War, specifically the battle of Jutland.

The 1921 fleet program was cancelled due to signing of the Washington Naval Treaty in 1922, which limited the size and armament of battleships to 35,000 long tons (36,000 t) and no gun bigger than 16 inches (406 mm).

In 1916 the US had declared its intention to create a Navy "second to none"; Congress had authorized the building of a large number of battleships and battlecruisers.

Two improved Revenge-class battleship hulls were rebuilt into the two Renown-class battlecruisers by the Royal Navy during the war.

Consequently, a series of designs was prepared of ships with displacements ranging from 53,100 to 44,500 long tons (54,000 to 45,200 t), the only limitations being the ability to use British dockyards and passage through the Suez Canal.

This reduction in size allowed the ship to dock anywhere that Hood could, and to pass through the Suez and Panama Canals.

The configuration was considered poorly armoured and a more compact design I3 - with machinery aft, magazines centrally disposed and the main armament forward - was drawn up by the DNC and presented in late 1920.

The J3 battlecruiser design had four geared steam turbine sets, each driving one propeller shaft, and arranged in two engine rooms.

They were designed to produce a total of 151,000 shaft horsepower (113,000 kW) at a working pressure of 200 psi (1,379 kPa) and temperature of 200 °C (392 °F) with superheat.

These weapons would have been very similar in construction to the BL 16-inch Mark I produced for the Nelson- class battleships [10] The J3s carried a secondary armament of 12 BL 6-inch Mark XXII guns in twin turrets amidships behind the main bridge structure and ahead of the Z turret.

They fired a 50-pound (23 kg) high explosive shell at a rate of eight to twelve rounds per minute up to a maximum ceiling of 32,000 ft (9,800 m), and effective range out to 16,160 yards (14,780 m).

[11] The ships were intended to carry four eight-barreled mountings for the 40-millimetre (1.6 in) QF 2-pounder gun (commonly known as a "pom-pom"), two abaft the funnels and two at the stern.